Radar surveillance is used in civil fields (e.g. as an air traffic aid) and in the military field (e.g. in defense of an airspace).
The coverage of a ground-based surveillance system is limited by the horizon and by orographic constraints, particularly at relatively low altitude; a spaceborne system, by utilizing artificial satellites, is not affected by such limitations and can cover a very large area and distribute surveillance data to many users.
The main functional and operating characteristics of airspace radar surveillance, to date at an early definition stage, are highlighted in a few recent publications, such as:
(1). W. J. Caima "Space-based Radar Application to Air Traffic Control", IEEE Internal. Radar Conference, Arlington, May 6-9, 1985 (IEEE Cal No. 85 CH 2076-8), pp 312--312.
(2) E. Brooknes: "Derivation of a Satellite Radar Architecture for Air Surveillance", IEEE Eascon - 1983 (IEEE Cat. No. 0531-6863/83), pp 465-475.
(3). G. Galati, G. Losquadro "Space-based Multifunction Radar Systems: Future Tool for Civilian and Military Surveillance" 42nd Symposium of the Guidance & Control Panel of the AGARD, Brussels 10-13 June 1986, pp 31.1-31.9.
As highlighted in article 3 above, radar surveillance systems may use primary radars, secondary radars, or both primary and secondary radars. The characteristics and operation of the primary and secondary radar, which are standard systems used in international civil aviation, are well known.
Spaceborne surveillance cannot be performed by simply carrying present techniques and technologies of primary and secondary radars on board artificial satellites because of two types of problems, both due to the very large distance (in the order of thousands of kilometers) between the radar antenna and the targets to be detected.
The first problem is that of the power required to be transmitted, particularly high for the primary radar, because the product of the square of the antenna effective area and power transmitted is proportional to the fourth power of range.
Therefore the need arises for a high-power transmitter and for an antenna with a very large effective area, or in other words, having high gain.
The second problem is that of the very high angle-accuracy required to achieve accurate location at long range, comprable to that achievable with ground-based systems operating at much shorter ranges. This last requirement too needs an antenna with a very narrow main lobe, and therefore very high gain.
The dimensions of antennae with such performance characteristics are particularly large (ranging from a few hundred to a few thousand times the radar operating wavelength). The radar operating wavelength is set by operating and mission requirements for the primary radar and is typically between 10 and 25 cm, and by International Civil Aviation rules for secondary radars, known as SSRs, set at 17.5 cm and 29.1 cm.
Previous techniques attempted to solve, at least partly, these problems by means of physically large antennae, unfoldable in space in differing configurations. Mechanical and electrical aspects make the use of such antennae very critical and pose a limit to the maximum dimensions obtainable, fixed at a few tens of meters per side, which does not provide complete satisfaction.